The broadest objective is to understand the visual system and how it works. Psychophysical techniques are used because of their non-invasiveness, and threshold measures are used because, with a minimum of assumption, they afford conclusions about the transmission and processing of information by the visual system. The first set observations focuses on interactions between signals elicited from the rod and cone systems of the retina. The second set is directed toward an understanding of the optical properties of receptors: their optical apertures, the distribution of their orientations, the distribution of light within them (waveguide modal patterns), and the relationship of these properties to the directional sensitivity of the retina (the Stiles-Crawford effect). A third set addresses the problem of how singleness of vision results from the two different retinal images. A fourth set takes advantage of a lapse (transient sensitization) of a mechanism that light-adapts the eye, in order to reveal some of the properties of this mechanism for light adaptation and the properties of the underlying system normally concealed by this mechanism. A final set of observations concerns the cause and nature of certain hallucinatory phenomena that occasionally interfere with psychophysical techniques of studying the visual system.